Dispersive relaxation dynamics of photoexcitations in a polyfluorene film involving energy transfer:: Experiment and Monte Carlo simulations

Time-resolved fluorescence spectroscopy is used to investigate relaxation of electronic excitations in films of pi -conjugated polymer 1 in the ps time domain. The position of the fluorescence band and its width are measured as a function of time and excitation energy. Both low (15 K) and room-temperature behavior are investigated. For high energy excitation, the fluorescence band shows a continuous red shift with time. The energy associated with the maximum of the fluorescence band E is proportional to log(t), with t being the time after excitation. For excitation in the tail of the lowest absorption band, the fluorescence remains stationary and selective excitation of a subset of chromophoric chain segments is possible. At intermediate excitation energy the time required for the excitations to make their first jump depends on the excitation energy and is longer at lower energy. At low temperature and high energy excitation the fluorescence bands are found to narrow with time, while for low energy excitation a broadening with time is observed. The experimental data are consistent with dispersive relaxation dynamics for the photoexcitations by incoherent hopping between localized states. Monte Carlo simulations are performed to obtain the average energy and the width of the energy distribution for an ensemble of photoexcitations in an energetically disordered molecular solid assuming Forster type energy transfer. A Forster radius R-0 similar to 30 Angstrom is found to give good agreement between experiment and simulations. In addition, the measurements indicate that for excitation energies >2.94 eV additional relaxation processes, ascribed to ultrafast intrachain vibrational relaxation, are operative.